1. Technical Field
The present disclosure relates to an antenna device mounted in a wearable device or the like.
2. Description of the Related Art
In recent years, smart devices represented by smartphones are rapidly becoming widespread. As further development, small wearable devices are attracting attention. Devices having a near field wireless communication function such as wristwatch-type devices used in association with smartphones and terminals having a cellular communication function have been already commercialized.
A reduction in size of an antenna is essential for achievement of practical use of such a wearable device. However, the performance of an antenna is proportional to the working volume of the antenna. Accordingly, the performance of an antenna deteriorates as the whole size of the antenna becomes small.
In many cases, cellular communication of a mobile phone such as a smartphone supports multiband communication. In order to produce a multiband antenna device, it is general to employ an antenna configuration in which a feed system is an unbalanced type. The unbalanced-type feed system allows an antenna electric current to be also distributed in ground (GND) of a circuit board. This achieves a wider bandwidth and a higher gain while securing the working volume of the antenna.
However, in a case where a multiband antenna is employed in a small wearable device, the performance of the antenna deteriorates even by using the above configuration since the whole size of the antenna becomes small.
As a solution to this problem of a small antenna device, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2013-539322 discloses an antenna in which a GND enlarging unit is added to the antenna configuration using an unbalanced-type feed system and in which GND of a circuit board and the GND enlarging unit are designed to have a length that is proportional to approximately ¼ of the wavelength of frequencies in a plurality of frequency bands. This allows the working volume of the antenna to look large, thereby achieving a wider bandwidth and a higher gain.
In general, in the case of the antenna configuration using an unbalanced-type feed system, an antenna electric current also strongly flows in GND of a circuit board. This increases the electric current distribution density of GND of the circuit board. That is, a smaller device has a smaller circuit board and therefore has a higher electric current distribution density. As a result, an SAR (Specific Absorption Rate) value becomes higher.
However, the antenna disclosed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2013-539322 is not designed in consideration of an SAR value although the size of the device is small.